1. Field of the Invention
The present invention relates to distance measuring equipment (DME) that measures the distance between an aircraft and a ground apparatus. More particularly, the invention relates to a DME ground apparatus that is provided on the ground.
2. Description of the Related Art
The distance measuring equipment (hereinafter referred to as DME apparatus) is a secondary radar system composed of an airborne apparatus and a ground apparatus. The airborne apparatus is mounted in an aircraft. The ground apparatus is provided on the ground and communicates with the airborne apparatus.
In the DME apparatus, the airborne apparatus has an interrogator, and the ground apparatus has a transmitting-receiving device called transponder.
The interrogator transmits interrogation pulses of UHF band (pair pulses) toward the transponder provided in the ground apparatus. The distance between the aircraft and the ground apparatus is measured from the time that elapses until the interrogator receives response pulses (pair pulses) from the transponder after it has transmitted the interrogation pulses. (See, for example, Japanese Patent No. 2,629,612.)
The interrogator sequentially transmits interrogation pulses to the ground apparatus, at random time intervals (though the number of pulses per second is prescribed, e.g., 30 pulses per second). The ground apparatus receives the interrogation pulse signal of a prescribed frequency from the airborne apparatus mounted in the aircraft. The ground apparatus demodulates the interrogation pulse signal, decoding the same, and imparts a preset system-delay time (e.g., 50 μs) to the interrogation pulse signal thus decoded. The ground apparatus then encodes the interrogation pulse signal, generating a response pulse signal. The response pulse signal is transmitted to the aircraft via a specific transmitting system.
On receiving the response pulses, the airborne apparatus mounted in the aircraft demodulates the response pulse signals and calculates the time that has elapsed from the transmission of the interrogation pulse signals to the reception of the response pulse signals. From the time thus calculated, the airborne apparatus generates distance data representing the distance between the aircraft and the DME ground apparatus, by using a prescribed algorithm.
The transponder can respond to the interrogations coming from a plurality of interrogators (mounted in aircrafts).
On receiving interrogation pulses, the DME ground apparatus transmits response pulses as described above. The DME apparatus is obligated to generate random response pulse signals, 700 or more pulses per second on average, to maintain the automatic gain control in the interrogator mounted in each aircraft, even if the DME ground apparatus receives no interrogation pulses from the aircraft.
The random response pulses are called squitter pulses. It is important that squitter pulses should be transmitted randomly so that they may as little correlated as possible with the interrogation pulses that are coming from the aircraft. (See, for example, Jpn. Pat. Appln. Laid-Open Publication No. 63-208782.)
Therefore, the DME ground apparatus has a decoder that receives the interrogation pulses transmitted from the interrogator mounted in the aircraft and decodes these pulses at random time intervals, by using an analog circuit that utilizes the randomness of noise, such as thermal noise made in the receiver provided in the transponder. (Although the time intervals are random, the decoding rate is fixed, for example 1000 pps (pulses pairs per second).
In other words, the transponder transmits a response pulse signal in place of a squitter pulse signal only when it receives an interrogation pulse signal. Hence, the DME ground apparatus keeps transmitting squitter pulse signals and response pulse signals at the rate of 1,000 to 2,700 pps in its normal operating state.
As described above, the transponder of the DME ground apparatus generates and transmits squitter pulses at random time intervals even if it receives no interrogation signals from any aircraft, for the purpose of, for example, maintaining the automatic gain control in the interrogator mounted in each aircraft.
Hitherto, squitter pulses have been generated by an analog circuit that utilizes the randomness of noise, such as thermal noise made in the receiver provided in the transponder. This is because squitter pulses should be transmitted at random time intervals. The analog circuit is susceptible to external noise, however. Consequently, the rate of transmitting the squitter pulses varies, possibly deviated from the appropriate transmission range.